The three channel development system used in electrophotographic printers has a development roller that moves developer containing toner into proximity with a primary imaging member, usually a photoconductor; and a first channel containing a feed auger, a second channel containing a second auger, a third channel containing at least a third auger, and possibly a fourth auger. The primary imaging member is used for forming an electrostatic image. The developer used in development systems of this type usually contains magnetic particles and marking particles. The marking particles are removed from the development system to form an image on the primary imaging member.
The flow of developer through the three channel development system is such that developer is fed from the third channel to a first end of the feed auger in the first channel. As the developer travels longitudinally down the length of the feed auger, a portion of the developer is fed transversely from the feed auger to the development roller to produce a layer of developer on the development roller. The remainder of developer in the first channel continues to travel longitudinally down the length of the feed auger.
To produce a uniform image, the layer of developer on the development roller should be uniform along its length. The developer that is fed to the development roller moves over the development roller and is not returned to the feed auger but instead drops into the second auger in the second channel. Consequently, the volume of developer in the first channel decreases along the length of the first channel in the direction of developer flow along the first channel.
Developer moves longitudinally in the same direction in both the first channel and the second channel, from the first end of the augers to the second end, which is at the rear or drive end of the development system. At the rear of the development system, the developer collected by the second channel and the remaining developer in the first channel are both dropped into the third channel. It is also at this point that replenishment marking particles are added to the developer to replace the marking particles that have been applied to the primary imaging member. The developer is moved longitudinally along the third channel by the third auger, or possibly by a third and forth auger acting together, toward the first end of the feed auger. The developer that has traveled the length of the third channel is fed to the first end of the feed auger in the first channel, so that the developer is cycled continuously from the first channel to the development roller, from the first and second channels to the third channel, and from the third channel to the first channel while the development system is running.
In comparison, two channel development system designs often have the characteristic that developer that has travelled over the development roller is dropped back into the channel from which it was fed to the development roller. Some of this developer will have had marking particles removed by the image. In other words, the concentration of marking particles in the developer is reduced as the developer is used for image development, returned to the feed auger, and subsequently travels down the feed auger of a two channel development system. As the toner concentration decreases, the developed mass and image density also decrease undesirably.
An advantage of the three channel design compared to a two channel design is that the marking particle concentration is maintained down the length of the first channel. However, the volume of developer in the first channel does not remain constant down its length, usually resulting in more developer on the development roller near the first end of the feed auger, where there is a relatively large volume of developer in the first channel. Near the second end of the feed auger, where there is a relatively small volume of developer, there is usually less developer on the development roller.
It is advantageous to have a constant mass flow of developer at any point along the entire length of the development roller as well as having a constant marking particle concentration in the developer that is presented to the primary imaging member via the development roller. Specifically, it is advantageous to have a means of maintaining the developer feed to the development roller despite the reduction in developer volume down the length of the first channel.